Amyloid beta protein starting pyroglutamate at position 3 is a major component of the amyloid deposits in the Alzheimer's disease brain

Crystal Structure of Papaya Glutaminyl Cyclase, an Archetype for Plant and Bacterial Glutaminyl Cyclases

Glutaminyl cyclases (QCs) (EC 2.3.2.5) catalyze the intramolecular cyclization of protein N-terminal glutamine residues into pyroglutamic acid with the concomitant liberation of ammonia. QCs may be classified in two groups containing, respectively, the mammalian enzymes, and the enzymes from plants, bacteria, and parasites. The crystal structure of the QC from the latex of Carica papaya (PQC) has been determined at 1.7A resolution. The structure was solved by the single wavelength anomalous diffraction technique using sulfur and zinc as anomalous scatterers. The enzyme folds into a five-bladed beta-propeller, with two additional alpha-helices and one beta hairpin. The propeller closure is achieved via an original molecular velcro, which links the last two blades into a large eight stranded beta-sheet. The zinc ion present in the PQC is bound via an octahedral coordination into an elongated cavity located along the pseudo 5-fold axis of the beta-propeller fold. This zinc ion presumably plays a structural role and may contribute to the exceptional stability of PQC, along with an extended hydrophobic packing, the absence of long loops, the three-joint molecular velcro and the overall folding itself. Multiple sequence alignments combined with structural analyses have allowed us to tentatively locate the active site, which is filled in the crystal structure either by a Tris molecule or an acetate ion. These analyses are further supported by the experimental evidence that Tris is a competitive inhibitor of PQC. The active site is located at the C-terminal entrance of the PQC central tunnel. W83, W110, W169, Q24, E69, N155, K225, F22 and F67 are highly conserved residues in the C-terminal entrance, and their putative role in catalysis is discussed. The PQC structure is representative of the plants, bacterial and parasite enzymes and contrasts with that of mammalian enzymes, that may possibly share a conserved scaffold of the bacterial aminopeptidase.

Crystal structures of human glutaminyl cyclase, an enzyme responsible for protein N-terminal pyroglutamate formation

N-terminal pyroglutamate (pGlu) formation from its glutaminyl (or glutamyl) precursor is required in the maturation of numerous bioactive peptides. The aberrant formation of pGlu may be related to several pathological processes, such as osteoporosis and amyloidotic diseases. This N-terminal cyclization reaction, once thought to proceed spontaneously, is greatly facilitated by the enzyme glutaminyl cyclase (QC). To probe this important but poorly understood modification, we present here the structure of human QC in free form and bound to a substrate and three imidazole-derived inhibitors. The structure reveals an alpha/beta scaffold akin to that of two-zinc exopeptidases but with several insertions and deletions, particularly in the active-site region. The relatively closed active site displays alternate conformations due to the different indole orientations of Trp-207, resulting in two substrate (glutamine t-butyl ester)-binding modes. The single zinc ion in the active site is coordinated to three conserved residues and one water molecule, which is replaced by an imidazole nitrogen upon binding of the inhibitors. Together with structural and kinetic analyses of several active-site-mutant enzymes, a catalysis mechanism of the formation of protein N-terminal pGlu is proposed. Our results provide a structural basis for the rational design of inhibitors against QC-associated disorders.

Amino-Truncated β-Amyloid42 Peptides in Cerebrospinal Fluid and Prediction of Progression of Mild Cognitive Impairment

Background: Early identification of patients with mild cognitive impairment (MCI) progressing to Alzheimer disease (MCI-AD) by use of biomarkers in cerebrospinal fluid (CSF) is an essential step toward improving clinical diagnosis and drug development. We evaluated whether different β-amyloid42 (Aβ42) peptides can add further information to the combined use of tau and Aβ1–42 for predicting risk of progression of MCI to AD.

Methods: We used xMAP® technology to simultaneously quantify different Aβ42 peptides modified at the amino terminus, tau, and phosphorylated tau (P-tau181P) in CSF. Aβ42 peptide concentrations were measured by use of immunoreactivity toward Aβ monoclonal antibodies [3D6 (Aβ42-3D6), WO2 (Aβ42-WO2), 6E10 (Aβ42-6E10), and 4G8 (Aβ42-4G8)]. The discriminant ability of the markers was evaluated by ROC curve analysis.

Results: The areas under the curves for the separation of MCI-AD from nonprogressing MCI (MCI-N) were significantly higher when we used Aβ42-3D6/Aβ42-WO2, Aβ42-3D6/Aβ42-6E10, or Aβ42-3D6/Aβ42-4G8 compared with Aβ42-3D6. In addition, differentiation of MCI-N from MCI-AD was improved by quantification of full-length Aβ1–42 (Aβ42-3D6) compared with Aβ42-WO2, Aβ42-6E10, or Aβ42-4G8. Several Aβ42 peptides truncated at the amino terminus (Aβ11–42 and Aβ8–42) were identified in CSF by surface-enhanced laser desorption/ionization time-of-flight technology.

Conclusion: The CSF markers tau, Aβ42 forms, and P-tau181P, when used as adjuncts to clinical diagnosis, have the potential to help identify AD pathology and could be a valuable asset for early AD diagnosis.

beta-amyloid is different in normal aging and in Alzheimer disease

The mechanism of neurodegeneration caused by beta-amyloid in Alzheimer disease is controversial. Neuronal toxicity is exerted mostly by various species of soluble beta-amyloid oligomers that differ in their N- and C-terminal domains. However, abundant accumulation of beta-amyloid also occurs in the brains of cognitively normal elderly people, in the absence of obvious neuronal dysfunction. We postulated that neuronal toxicity depends on the molecular composition, rather than the amount, of the soluble beta-amyloid oligomers. Here we show that soluble beta-amyloid aggregates that accumulate in Alzheimer disease are different from those of normal aging in regard to the composition as well as the aggregation and toxicity properties.

Massive CA1/2 neuronal loss with intraneuronal and N-terminal truncated Abeta42 accumulation in a novel Alzheimer transgenic model

Alzheimer's disease (AD) is characterized by a substantial degeneration of pyramidal neurons and the appearance of neuritic plaques and neurofibrillary tangles. Here we present a novel transgenic mouse model, APP(SL)PS1KI that closely mimics the development of AD-related neuropathological features including a significant hippocampal neuronal loss. This transgenic mouse model carries M233T/L235P knocked-in mutations in presenilin-1 and overexpresses mutated human beta-amyloid (Abeta) precursor protein. Abeta(x-42) is the major form of Abeta species present in this model with progressive development of a complex pattern of N-truncated variants and dimers, similar to those observed in AD brain. At 10 months of age, an extensive neuronal loss (>50%) is present in the CA1/2 hippocampal pyramidal cell layer that correlates with strong accumulation of intraneuronal Abeta and thioflavine-S-positive intracellular material but not with extracellular Abeta deposits. A strong reactive astrogliosis develops together with the neuronal loss. This loss is already detectable at 6 months of age and is PS1KI gene dosage-dependent. Thus, APP(SL)PS1KI mice further confirm the critical role of intraneuronal Abeta(42) in neuronal loss and provide an excellent tool to investigate therapeutic strategies designed to prevent AD neurodegeneration.

Glutaminyl cyclases unfold glutamyl cyclase activity under mild acid conditions

N-terminal pyroglutamate (pGlu) formation from glutaminyl precursors is a posttranslational event in the processing of bioactive neuropeptides such as thyrotropin-releasing hormone and neurotensin during their maturation in the secretory pathway. The reaction is facilitated by glutaminyl cyclase (QC), an enzyme highly abundant in mammalian brain. Here, we describe for the first time that human and papaya QC also catalyze N-terminal glutamate cyclization. Surprisingly, the enzymatic Glu(1) conversion is favored at pH 6.0 while Gln(1) conversion occurs with an optimum at pH 8.0. This unexpected finding might be of importance for deciphering the events leading to deposition of highly toxic pyroglutamyl peptides in amyloidotic diseases.

Decreased level of brain acetylcholine and memory disturbance in APPsw mice

To clarify whether amyloid beta protein (Abeta) amyloidosis induces a disturbance of cholinergic system leading to long-term memory deficits, we continuously examined memory disturbance using the passive-avoidance task, and measured Abeta burden and concentrations of acetylcholine in the brain of APPsw transgenic mice. Repetitive retention trials of the passive-avoidance task showed that the long-term memory impairment in APPsw mice appeared from approximately 7.75 months old and progressively advanced. Significant decreases in acetylcholine levels were found in the brains of 10-month-old mice. A few senile plaques appeared in the cerebral cortex and the hippocampus at 8 months old, and increased in size and number with aging. The concentrations of brain Abeta40/42(43) gradually increased from 8 months old and exponentially increased thereafter. Advance of long-term memory disturbance was closely correlated with Abeta40/42(43) burden. These findings suggested that Abeta accumulation induced long-term memory impairment and disturbance of the cholinergic system, and that the passive-avoidance task and measuring acetylcholine were useful methods for evaluating this mouse model as well as Abeta accumulation.

Pyroglutamate-modified amyloid beta-peptides--AbetaN3(pE)--strongly affect cultured neuron and astrocyte survival

N-terminally truncated amyloid-beta (Abeta) peptides are present in early and diffuse plaques of individuals with Alzheimer's disease (AD), are overproduced in early onset familial AD and their amount seems to be directly correlated to the severity and the progression of the disease in AD and Down's syndrome (DS). The pyroglutamate-containing isoforms at position 3 [AbetaN3(pE)-40/42] represent the prominent form among the N-truncated species, and may account for more than 50% of Abeta accumulated in plaques. In this study, we compared the toxic properties, fibrillogenic capabilities, and in vitro degradation profile of Abeta1-40, Abeta1-42, AbetaN3(pE)-40 and AbetaN3(pE)-42. Our data show that fibre morphology of Abeta peptides is greatly influenced by the C-terminus while toxicity, interaction with cell membranes and degradation are influenced by the N-terminus. AbetaN3(pE)-40 induced significantly more cell loss than the other species both in neuronal and glial cell cultures. Aggregated AbetaN3(pE) peptides were heavily distributed on plasma membrane and within the cytoplasm of treated cells. AbetaN3(pE)-40/42 peptides showed a significant resistance to degradation by cultured astrocytes, while full-length peptides resulted partially degraded. These findings suggest that formation of N-terminally modified peptides may enhance beta-amyloid aggregation and toxicity, likely worsening the onset and progression of the disease.